The present invention relates to a dielectric ceramic composition, especially a dielectric ceramic composition suitable for forming resonators, etc. used in the microwave, and millimeter wave ranges including wavelengths of several hundred MHz to several tens GHz in particular, a dielectric ceramic material and its production method, and a dielectric device and its production method.
To meet recently increasing demands for mobile communications equipment represented by earphones, portable phones, and satellite communications as well as demands for ever-higher intelligence and performance, it is now strongly required to achieve size reductions and high performance regarding microwave devices used on mobile communications systems. By the term "microwave device" used herein is intended a dielectric device made up of a dielectric ceramic material, which has main applications in the microwave, and millimeter wave ranges and is used to form resonators, etc.
Such a microwave device is mounted on a substrate by surface mount technologies, and is required to include leading electrodes having good wettability by solder and less exposed to a solder attack upon mounted on the substrate, and stand up to mechanical, and thermal impacts upon mounted on the substrate.
The size of a microwave device built up of a conductor and a dielectric material is substantially inversely proportional to the square root of a dielectric constant .epsilon. of the dielectric material used, and Q, an index to device performance, is defined by the reciprocal of a loss factor. Although some correction is necessary depending on device architecture, Q is chiefly determined by EQU 1/Q=1/Qc+1/Qd
Here Qc is the reciprocal of a loss factor of the conductor portion and Qd is the reciprocal of a loss factor of the dielectric portion. To obtain a small-size yet low-loss microwave device suitable for surface mounting, therefore, it is required to increase the dielectric constant .epsilon. of the dielectric material and increase the reciprocals Qc and Qd of the loss factors of the conductor and dielectric portions, respectively.
Nickel plating is carried out to prevent a solder attack on electrodes upon mounting, and tin, lead and other plating is performed to improve wettability by solder. It is thus required to prevent product defects and degradation of device performance, which may otherwise be caused by popping, corrosion, etc. due to the entrance of a plating bath into devices at the nickel, tin, lead, and other plating step.
Here let f represent the frequency used. Qc decreases as f increases, and Qc decreases again as .rho. increases. Here .rho. is the specific resistance of a conductor that is a constant intrinsic to a conductor material. Empirically, it is known that f.Qd=C holds in the microwave, and millimeter wave ranges. Here C is a constant intrinsic to a dielectric material, called a Qf product. Qd is in inverse proportion to f but in direct proportion to C. To increase the Q of a device, therefore, it is most effective to decrease the .rho. of the conductor material. This effect is particularly true as the frequency f becomes high. The second best is to increase the C of a dielectric material.
However, a substance having the lowest specific resistance .rho. in a temperature range of -50 to 125.degree. C. is silver. To increase the Q of a device, therefore, it is most effective to use silver for the conductor material, and the second best is to use a dielectric material having a large Qf product. In view of obtaining a low-loss device, it is here to be noted that even a dielectric material having a large Q value and a high dielectric constant is of industrially low value unless it can have a structure integral with silver having the lowest specific resistance .rho..
So far, Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3, Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3, etc. have been known as microwave dielectric materials having a high dielectric constant and a large Q value in the microwave, and millimeter wave ranges. However, these materials are found to have a sintering temperature of as high as 1,300.degree. C. or higher, and so cannot be co-sintered with silver. The sintering temperature of the materials may be lowered by the addition of glass thereto, but the resulting materials are no longer practical because their dielectric constant is as low as about 10.
On the other hand, U.S. Pat. No. 5,459,115 (EP 0 589 441 A1) discloses a dielectric ceramic composition represented by EQU (Pb.sub.1-x Ca.sub.x).sub.1+y {(Fe.sub.1/2 Nb.sub.1/2).sub.1-2 (Fe.sub.2/3 W.sub.1/3).sub.z }O.sub.3+y
where x, z and y are EQU 0.43&lt;x.ltoreq.0.63 EQU 0.0&lt;z.ltoreq.0.5 EQU 0.0.ltoreq.y.ltoreq.0.20
However, this composition cannot be co-sintered with silver because of its high sintering temperature of the order of about 1,000 to 1,200.degree. C., although its dielectric constant is of the order of 90 or higher. It is thus impossible to fabricate a device of the structure wherein a low-loss silver conductor is embedded in a dielectric material or, in another parlance, it is difficult to obtain a device having a large Q value.
U.S. Pat. No. 5,565,391 (EP 0 625 492 A1) discloses a dielectric ceramic material having as a major phase a composition comprising lead, calcium, tungsten, iron and niobium in the form of oxides and represented by EQU (Pb.sub.1-z Ca.sub.z) (W.sub.s Fe.sub.t Nb.sub.u)O.sub.3
where, on condition that s+t+u=1, z, s, t and u are EQU 0.3.ltoreq.z.ltoreq.0.9 EQU 0.01.ltoreq.s.ltoreq.0.2 EQU 0.5.ltoreq.t.ltoreq.0.6 EQU 0.2.ltoreq.u.ltoreq.0.49
This dielectric ceramic material has a high dielectric constant of the order of 90 or higher and a sintering temperature lower than the melting point of silver. However, no sufficient density is obtained unless the sintering temperature is elevated, not only resulting in electrode defects due to the evaporation of silver, excessive sintering, and the reaction of silver with the dielectric ceramic material, but also ending up with defects due to the entrance of a plating bath into a device, frequency variations, and Q value drops. To meet recently increasing demands for size reductions of communications equipment in the microwave, and millimeter wave ranges, there are strong demands for further size reductions of microwave devices by dielectric constant increases.